bootm: set max decompression size for LZO

The LZO decompressor wasn't initializing the maximum output size, which
meant it would fail to decompress most of the time.

Reported-by: Matthias Weißer <weisserm@arcor.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Matthias Weißer <weisserm@arcor.de>
Acked-by: Simon Glass <sjg@chromium.org>